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It's disputed, if the linear, no threshold model is adequate. The problem is, 1:5 people die of cancer anyway and the people getting fullbody CTs tend to be either old and die rather soon, or are people who had cancer at some point to begin with. Really hard to find a signal there. The best data is still from the nuclear accidents/bombings.

Cells can repair (maybe faulty) a certain amount of damage, but may suicide when too much is broken (double-strand breaks). The amount and type of ROS generated by ionizing radiation also depends on your antioxidant state and how well tissue is saturated with oxygen (more ROS if you exercised before exposure). Generated ROS are a significant factor in cell damage, it's not just direct DNA hits. Some ROS can last for weeks and travel across cells to fuck things up.

I think we can confidently say, lowish radiation exposure is: not great, not terrible ;)